Switching element provided with a foil construction

ABSTRACT

A switching element provided with a foil construction includes a first supporting foil and a second supporting foil that are kept at a distance from one another via a spacer. The spacer has a recess in at least one active area of the switching element. A contact arrangement includes at least two electrodes and is arranged in the active area of the switching element so that an electrical current between the electrodes is established when both foil layers are pressed together. A structure made of an electrically nonconducting material is placed inside the active area of the switching element and prevents the electrodes from coming into contact in the area of the structure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is entitled to the benefit of and incorporates byreference in their entireties essential subject matter disclosed inInternational Application No. PCT/EP02/09536 filed on Aug. 27, 2002 andLuxembourg Patent Application No. 90 835 filed on Sep. 19, 2001.

FIELD OF THE INVENTION

The present invention relates to a switching element, in particular afoil-type switching element.

BACKGROUND OF THE INVENTION

Foil-type switching elements, such as membrane switches, foil pressuresensors or similar ones, in general include at least two essentiallyelastic foil layers arranged at a certain distance to one another. Thisis, for example, achieved by means of a spacer which is arranged aroundthe active region of the switching element and on which the two foillayers are adhered with their respective borders. In the active regionof the switching element, different contact arrangements are applied onthe foil layers, an electrical contact being established between thearrangements when the two foil layers are compressed, so that theswitching element is triggered. When the pressure is released from thefoil layers, these in turn are restored to their spaced position due totheir elasticity and the electrical contact between the various contactarrangements is interrupted.

Such switching elements have a very good response characteristic whichcan moreover be adapted to the respective purpose of application by thedesign of the elastic foil layers. Such a switching element further hasa very low assembly height and is in particular characterized by themultifarious possibilities of designing the command button. This makessuch switching elements particular suitable for the use in fields wheresmall structural dimensions and a flexible design of the command buttonsis required.

Due to these properties, such switching elements have meanwhile found awide application in almost all fields of engineering. A specificapplication of such switching elements relates to seat occupancy sensorsin vehicles. Such seat occupancy sensors include a multitude ofindividual switching elements arranged across a seating surface of aseat in the vehicle seat. The switching elements are, for example,arranged between the seat foam and the seat cover.

Especially this location for employing the switching elements causeshowever problems for some seats. In some seats, in particular in case ofleather fittings, the tension in the seat cover is indeed so high thatsome of the switching elements are already activated by the seat coverand are thus triggered even if the seat is not occupied. This is aso-called preload. Such a triggering caused by the position of assemblycan be principally avoided by a corresponding design of the carrierfoils, e. g. by using less flexible carrier foils having a highermodulus of elasticity. However, the use of less flexible carrier foilssimultaneously results, in particular with pressure sensors, in adeterioration of the dynamics of the switching element.

From the U.S. Pat. No. 4,382,165, a foil-type switching element is knownwhich can be employed in keyboard pads. The carrier foils of theswitching element are separate and electrically insulated, by a spacerof dielectric material which is directly applied on one of the carrierfoils with a predetermined pattern. The predetermined pattern comprisesopenings which enable a local electrical contact between contactsurfaces applied on the two foils, the spacer essentially keeping thefoils separate.

The U.S. Pat. No. 4,594,482 discloses an input pad having twotransparent layers, at least one of which is flexible. Each layercarries an electrically conductive film on one of its surfaces. Bothlayers are disposed at a certain distance one to another such that therespective films are facing each other. A first spacer is located at theperiphery of the thus attached layers for separating them. Additionalpunctiform spacers are distributed across the area of the input fieldand applied on one of the layers.

OBJECTS AND SUMMARY OF THE INVENTION

It is consequently the object of the present invention to propose afoil-type switching element which enables an improved adaptation of theswitching characteristic.

Such a foil-type switching element comprises a first carrier foil and asecond carrier foil arranged at a certain distance one to another bymeans of a spacer, wherein the spacer comprises at least in an activeregion of the switching element a recess, and wherein a contactarrangement with at least two electrodes is arranged in the activeregion of the switching element such that an electrical contact betweenthe electrodes is established when the two foil, layers are compressed.At least one of the two electrodes comprises a layer of apressure-sensitive material, such that the electric resistance betweenthe electrodes depends on the exerted pressure when the two foil layersare pressed together. According to the invention, a structure of aAelectrically nonconducting material is arranged in the active region ofthe switching element for preventing a contact of the electrodes in theregion of the structure.

When the two carrier foils are pressed together, a mechanical contactbetween the various layers is first established at the location of thestructure of nonconducting material. This merely mechanical contactoccurs at a pressure at which in a conventional switching element anelectrical contacting of the electrodes is already effected. Only whenthe pressure on the switching element is further increased, theelectrodes in the switching element according to the invention areelectrically contacted in the region around the structure and theswitching element is triggered. By incorporating the structure ofdielectric material into the active region of the switching element, thethreshold of the switching characteristic, i.e. the minimum load atwhich the switching element will be triggered, is consequentlyincreased. This mechanical influence on the switching characteristic ofthe switching element has no effect on the flexibility of the carrierfoils and accordingly hardly influences the further dynamics of theswitching element above the threshold. This is of special importance inparticular with foil pressure sensors.

The person skilled in the art will appreciate that the lateraldimensions of the structure are to be essentially smaller than therespective corresponding dimensions of the active region of theswitching element, so that the switching element can be triggered. Itwill be furthermore appreciated that due to an appropriate design of theshape and dimension of the structure, the threshold of the switchingelement can be adjusted over a wide region. Accordingly, it is possibleto optimally adapt the switching characteristic of such a switchingelement over a wide region to the respective desired application.

The structure of electrically nonconducting material, i.e. of dielectricmaterial, can, for example, comprise at least one punctiform elevation.This punctiform elevation can, for example, be applied centrally in theactive region of the switching element onto one of the carrier foils oronto the contact arrangement. If the structure consists of severalpunctiform elevations, these are, for example, arranged distributedacross the area of the active region of the switching element in asymmetric arrangement.

In an alternative embodiment, the structure comprises at least oneannular elevation which is/are preferably arranged concentrically to theactive region of the switching element. A further variant relates to,for example, one or several linear elevations which are preferablyarranged in radial orientation, for example at the edge of the activeregion. Here, too, a symmetric arrangement is preferred for achieving anoptimised switching characteristic.

Naturally, the structure of dielectric material can also comprise anycombination of the above mentioned shapes.

In order to ensure an adequate response characteristic of the switchingelement, the height of the structure is to be selected in general to beessentially smaller than the height of the spacer, i.e. than thedistance between the two carrier foils. This is the only way ofachieving that, when the mechanical contact is established, at least oneof the carrier foils has already been subjected to a sufficiently largedeflection for causing an electrical contact between the electrodes whenthe pressure on the switching element is further increased. Only in thisway, a good dynamics of the switching element is ensured above thethreshold of the switching element. In general, therefore, the height ofthe structure should be smaller than one third of the height of thespacer, preferably even smaller than one sixth of the height of thespacer.

It should be noted that the present invention is applicable to allcommon types of foil-type switching elements. The switching element can,for example, operate in the so-called “through mode”. In such aswitching element, the contact arrangement comprises two electrodes, afirst electrode being applied on the first carrier foil and a secondelectrode being applied on the second carrier foil opposite the firstelectrode. If the switching element moreover is to be employed as foilpressure sensor, at least one of the two electrodes comprises a layer ofa pressure-sensitive material, such that the electric resistance betweenthe electrodes depends on the pressure exerted on the switching elementwhen the two foil layers are pressed together.

It should be noted that as pressure-sensitive material one frequentlyuses semi-conductor materials, which either have a specific resistancechanging depending on the pressure or the surface resistance of whichwith respect to an electrode is changed depending on the exertedpressure.

In an alternative embodiment, the switching element operates in theso-called “shunt mode”. In such an embodiment, the contact arraycomprises two electrodes and a contact element, the two electrodes beingapplied on one of the two carrier foils in a spaced relation and thecontact element of the first and the second electrodes being appliedoppositely on the other one of the two carrier foils. When the switchingelement is triggered, the contact element is pressed onto the twoelectrodes, so that an electrical contacting of the two electrodes iseffected across the contact element.

Such a switching element, too, can be designed as a foil pressuresensor, wherein at least one of the two electrodes or the contactelement comprises a layer of a pressure-sensitive material, such thatthe electric resistance between the electrodes depends on the pressureexerted on the switching element, when the two foil layers are pressedtogether.

It should be noted that the two pressure sensor types are known underthe designation of “Force Sensing Resistors”, or FSRS.

The structure of dielectric material can be either applied directly ontoone of the two carrier foils or else onto one of the electrodes or thecontact element. It should be noted in this context that with astructure with several elements these elements can be distributed overvarious ones of the mentioned elements. For example, one half of theelements of the structure can be applied onto each of the two carrierfoils.

It should be noted that the application of the structure ofnonconducting material onto the carrier foils or the electrodes or thecontact element is preferably effected by printing, for example in ascreening process.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, one embodiment of the invention is described withreference to the enclosed Figures, wherein:

FIG. 1 shows a section through an embodiment of a switching element (a)according to the invention as well as a plan view of the active regionof the switching element (b);

FIG. 2 shows various embodiments of the structure of nonconductingmaterial.

FIG. 1 shows under a) a section through a foil-type switching element.

DETAILED DESCRIPTION OF THE INVENTION

The switching element 10 comprises a first and a second carrier foil 12and 14 being laminated together by means of a spacer 16, for example adouble-sided bonding sheet. In the active region 18 of the sensor 10,the spacer 16 comprises a recess 20, so that in this region, the twocarrier foils 12 and 14 are facing each other at a distance.

In the active region 18 of the sensor, contact arrangements 22 and 24are arranged on the inside of the carrier foils 12 and 14, between whicharrangements an electrical contact is created when the two carrier foilsare pressed together. The contact arrays 22 or 24 respectively can, forexample, comprise electrode structures, at least one of the contactarrangements additionally comprising a layer of a pressure-sensitivematerial. The contact arrangements are, for example, applied onto thecorresponding areas of the carrier foils in a screen printing processbefore the carrier foils are laminated.

In the represented switching element 10, a structure 26 of a dielectric,i.e. electrically nonconducting material, is applied in the centralregion of the active region. In the represented embodiment, thestructure 26 comprises a punctiform elevation which first establishes amechanical contact between the two foils when the two carrier foils arepressed together, before an electrical contacting of the electrodes iseffected.

FIG. 2 shows in a plan view onto an active region of a switching elementvarious possible embodiments of such a structure of a dielectricmaterial. a) shows a punctiform elevation as represented in FIGS. 1 a)and b). FIG. 2 b) shows an embodiment with several punctiformelevations. c) represents an-embodiment with two annular structures. Thepartial drawings d) and e) show various possible embodiments of thestructure with linear elevations extending radially from the edge of theactive region towards the centre. f) shows a combination of annular andlinear elements.

It should be noted that, apart from the shown symmetric arrangements, incertain cases even an unsymmetrical arrangement of various elements ofthe structure is conceivable.

LIST OF REFERENCE NUMERALS

-   10 Switching element-   12, 14 Carrier foils-   16 Spacer-   18 Active region of the switching element-   20 Recess-   22, 24 Contact arrangement-   26 Structure of dielectric material

1. A foil-type switching element comprising: a first carrier foil and asecond carrier foil, said first carrier foil and said second carrierfoil being arranged at a certain distance one to another by means of aspacer, said spacer having a height corresponding to said distance, saidspacer including a recess at least in an active region of the switchingelement such that said spacer is arranged around said active region; acontact arrangement with at least two electrodes, the electrodes of saidcontact arrangement being arranged in the active region of the switchingelement in such a way that an electrical contact is established betweenthe electrodes when the carrier foils are pressed together, at least oneof said electrodes including a layer of a pressure-sensitive material,such that the electric resistance between the electrodes depends on thepressure exerted on the switching element when the carrier foils arepressed together; and a structure of an electrically nonconductingmaterial having a first side and a second side corresponding to saidfirst and second carrier foils, said structure arranged in the activeregion of the switching element, said structure of electricallynonconducting material having a height less than said spacer such thatmechanical contact is established on both of said first and second sidesonly after at least one of the first and second carrier foils issubjected to a deflection, said structure preventing a contacting of theelectrodes in the region of the structure.
 2. The switching elementaccording to claim 1, wherein the structure comprises at least onepunctiform elevation.
 3. The switching element according to claim 1,wherein the structure comprises at least one annular elevation.
 4. Theswitching element according to claim 1, wherein the structure comprisesat least one linear elevation.
 5. The switching element according toclaim 1, wherein a height of the structure is smaller than one third ofa height of the spacer.
 6. The switching element according to claim 1,wherein the contact arrangement comprises two electrodes, a firstelectrode being applied on the first carrier foil and a second electrodebeing applied on the second carrier foil opposite the first electrode.7. The switching element according to claim 1, wherein the contactarrangement comprises two electrodes and a contact element, the twoelectrodes being applied on one of the two carrier foils in a spacedrelation and the contact element being applied oppositely of the firstand the second electrodes on the other one of the two carrier foils. 8.The switching element according to claim 7, wherein at least one of thetwo electrodes or the contact element comprises a layer of apressure-sensitive material, such that the electric resistance betweenthe electrodes depends on the pressure exerted on the switching elementwhen the two foil layers are pressed together.
 9. The switching elementaccording to claim 7, wherein the structure of electricallynonconducting material is applied on the contact element.
 10. Theswitching element according to claim 1, wherein the structure ofelectrically nonconducting material is applied on one of the electrodes.11. The switching element according to claim 1, wherein the structure ofnonconducting material is printed on the carrier foils or theelectrodes.
 12. The switching element according to claim 7, wherein thestructure of nonconducting material is printed on the contact element.13. The switching element according to claim 7, wherein the structure ofnonconducting material is printed on the carrier foils or theelectrodes.